1. Field of the Invention
This invention relates to a method of controlling a ballast for a high intensity discharge (HID) lamp, and more particularly, to a method of controlling a ballast to enable the HID lamp to operate in its dedicated rated power and a related system.
2. Description of Related Art
A high intensity discharge (HID) lamp, such as a metal halide lamp, high pressure sodium lamp and a mercury vapor lamp, employs a gas discharge principle to generate visible light. The HID lamp has the advantages of compact size, high light emitting efficiency and broad power range, e.g. from a few watts to ten of thousands of watts, and is becoming one of the most popular lighting devices in the market.
A HID lamp cannot operate without a ballast. However, an existing ballast is designed to be applied to a HID lamp operating in specific rated power, and the HID lamp has to operate with its own dedicated ballast in order to operate normally. Since there are many models of HID lamps in the market and these HID lamps have diverse rated power, a variety of standards of ballasts, each of which controls only one model of HID lamp, have to be brought to the market accordingly, which increases the cost for the manufacturers and dealers in manufacturing backup, scheduling and product stock. Further, in selecting a ballast that matches with the rated power of a HID lamp, it is easily to erroneously select a ballast that does not match with the HID lamp, since HID lamps, even operable in various rated power, use the same lamp stand. If the ballast is erroneously selected and does not match with the HID lamp, and since the existing ballast is designed to control the HID lamp by driving the current or power of the HID lamp, once the HID lamp is turned on and operates in a stable process, the HID lamp still has similar performance, regardless of the variation of the rated power. For example, a metal halide lamp of a low rates less than 150 watts, when operating in the stable process, has a HID lamp voltage within a range from 80 to 90 voltages. Even if under an over-power and modifying process, the metal halide lamp still has very little variation on the HID lamp voltage. Therefore, the ballast functions normally, and the HID lamp continues to operate in the power driven by the ballast, but the life span of the HID lamp is greatly affected. Therefore, after the HID lamp is driven by the current ballast to enter the stable process, no message is presented for identification to reselect a matched ballast.
Under the premise that the lamp stands have such a high compatibility, it is demanding for research a general ballast applicable to a HID lamp operable in various rated power, in order to reduce the cost, prevent from erroneous selection of the ballast, and increase the life span of the HID lamp.